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Disc Sander Build

I've had this 1/3 HP motor laying around for the last few years. I kept it when we threw out out previous furnace, because the motor itself was less than 2 years old, and I thought it might be useful in something. The more I thought about things, the more I thought that a twelve-inch disc sander would be a good project.

The problem with the motor is that it is "only" 1/3 horsepower, which is pretty minimal.

As I looked around various websites and forums and so on, I found several examples of people making 12" disc sanders. But in all of that searching, I only found one person who had uses a 1/3HP motor In fact, he also used a furnace motor. Unfortunately, he only showed the build, and did not post any later follow up notes about how well it actually performed, and how he used it, and so on.

All the other DIY disc sander projects that I found were built using 1HP motors.

But in the end, I decided to just go ahead and give it a try. I had nothing to lose, as I was not going to go out and buy a motor or parts. As well, I thought it would be a fun exercise. Even if it turned out to NOT be strong enough, I would still have had fun and learned something.

The first order of business is to figure out how to mount a 12" disc to the motor -- without spending money! Generally the way to do that is to screw it to the motor pulley.

My motor came with a split pulley -- the pulley was made up of two parts that screwed together so that you could adjust it for wider or narrower belts. I took it completely apart and reversed the one side, which gave me a flat surface to mount to a mounting block. (I would in turn mount the 12" disc to that mounting block.)

There was a small section at the middle that protruded from the pulley, so I measured it, and found a comparable forstner bit and then drilled a shallow recess in the center of the mounting block, so that it could then mount flush to the pulley.

I also had to cut a small slot halfway through the mounting block, so that I could still fit a hex key (Allen key) into the pulley to secure it to the motor shaft.

I then drilled holes through the pulley, and attached the mounting block with four countersunk screws.

Before going further, I then temporarily wired up a cord and tested the motor. The motor ran smooth and quiet. More importantly, the mounting block did not wobble at all. It appear to stay perfectly true and flat.
I then moved on to make a 12" disc and attach it to the mounting block. I used a piece of 3/4" thick (19mm) Baltic birch plywood. I used it because (a) I had it and (b) Baltic birch is high quality plywood with no voids, so I was hoping it would have a consistent density, and (c) it was pretty heavy, and I thought the extra mass would be good for the sanding disc.
I screwed it to the mounting block with four counter-sunk screws through the face.
However, the first time I plugged the motor in with the disc attached, it nearly jumped off the bench. I was fortunate that I had attached a clamp. But even that was not sufficient and I quickly unplugged the motor and held it down.
First, I determined that the disc was not quite centred on the mounting block. I fixed that by fixing a belt sander and rotating the disk against the sandpaper. This gave me a perfectly round circle. But it still ran very rough.

It turned out that there was a small wobble in the disc, which was enough to shake the motor. I tried removing the disc and mounting block and the motor ran smooth and quiet.

As an experiment a grabbed a piece of MDF shelving that I had available. It was absolutely flat. I traced the previous disc and cut out a new 12" disc and mounted that on the mounting block. The motor now ran much smoother.

I then mounted the motor on a piece of wood, and put two more pieces under that as legs. This lifted the motor about 5" up off the bench, so that the disc cleared the bench.

The next step was to build a dust shroud / safety shroud to fit behind and over the disk. I don't want to get fingers or anything else caught behind the spinning disc! I pulled out more of the scrap MDF shelving and used the first 12" disc as a template for routing a semi-circle-topped arch to fit behind the disk and over the motor shaft.

Warning: Math Ahead!

Making the archway over the top of the dust shroud was tricky. I could just nail some blocks into place, but then I would have gaps between each block. I wanted to figure out how to rip the blocks with an angle on each side, so that they would fit flush together, like stones in an archway, or like staves in a barrel.

Here is how I figured this out. The arch is a half circle. So, I measured the diameter of the arch (12-3/8"), and multiplied that by pi to give me the circumference (38.87"), and then divided that in half. I now had the length of the arch (19.438").

I thought that half-inch wide pieces would be a good size, so I figured I would need 39 pieces, each a half inch wide, to cover that 19.4" arch.

There are 180 degrees in a half circle, so 180 divided by 39 pieces gives me a figure of 4.61 degrees. So I concluded that I needed to tilt my table saw blade by 4.6 degrees when ripping pieces to make up the arch of the shroud.

(how's my math?)

With all the pieces ripped and cut down to length, I started gluing and nailing them around the arch. Each piece was glued to the arch, and to it's neighbour.
The outside of the arch looked great. All the pieces were nice and ight. The inside was full of gaps. Either my math is off, or my tool settings were a bit off, or something else went wrong. (I counted and there are only 35 pieces on the arch , so I know that I messed up a bit with the width of my pieces.)
This was easily fixed, as I mixed up some epoxy, thickened it to a paste with some silica powder, and packed it in all along the inside. The results were a bit ugly, but I don't care about that, as it only faces the backside of the disc. The important thing was that the dust/safety shroud was strong, and complete, and could now be mounted between the disc and the motor.
I next made a simple sanding platform. I just screwed some plywood together for a fixed non-moving platform. In part, this is still just an experiment, so I do not want to make a complicated structure here. But also, I really am not sure I'll need anything more than this. Time will tell.
I'm now in the home stretch. I wanted to add some dust collection, so I added a bottom to the sander. This would also give me a way to clamp it to the workbench when using. As well there is a lower piece just in front of the disk, and a back.
I traced my shop vac hose on the back and cut out a matching hole using the scroll saw. The dust hose now fits perfectly into the back, so hopefully it will suck the dust out right as it is thrown off the bottom of the disc.
Mounting a 12" adhesive sanding disc. This is a 60 grit pad, and it was my only out-of-pocket expense for this project.
I had some cork available, so I cut out some 2" squares which I glued to the bottom to use as feet. That will help keep the sander from slipping around on the bench.
I used an old light switch for an on/off switch on the side of the unit.
After that, I tested out the sander on softwoods and hardwoods. It seems to work just fine. With the shop vac attached I could not detect any dust being thrown around.

And that is about all for this experiment.

I've managed to put together a functional disc sander, and now I need to just live with it and use it for a month or so. That will give me some time to see how I like it, how I use it, and how it fits into my shop and workflow.

I plan to update this page in a few months with those results.

I can already thing of some future work that I may want to do. First, I could paint it. Second, I probably should built a dust shroud/enclosure for the motor. Also, remember that the motor is reversible... I still might hook up another switch so that I can switch between clockwise and counter-clockwise rotation.

Why would I do that? Well, mostly just because I can. I have come across one comment online from someone who claimed that his sandpaper lasted longer when he could sand in both directions. But I might just not bother also.

Full disclosure: there is still a small wobble in the disc -- just under 1/16". I do not know if this is the fault of the disc, the mounting block, the pulley, or the motor itself. I am going to wait and see if this is a problem or not in actual use.

Net cost of this whole project was $5 (plus tax) for the 12" sanding disc. Everything else, starting with the surplus motor, was just shop scraps.

I'm pretty happy with this experiment!


Thanks for reading!

See Also:


Tool Cabinet

Tool Tote Organizer

Fliptop Stand: Tour and Teardown

Fixing My Tablesaw Stand